The field of the invention relates to a loudspeaker including a compression chamber, a first electroacoustic transducer disposed inside the compression chamber, a horn and a second electroacoustic transducer disposed outside the compression chamber.
U.S. Pat. No. 4,138,594 teaches a small dimension low frequency loudspeaker which includes a folded exponential horn which provides a unitary curved sound path from an electroacoustic transducer at the throat of the horn to a volume into which sound is radiated at the mouth of the horn. The length of the horn is such that, at an exponential rate of expansion between the throat and the mouth, the mouth, when it is bounded by at least one planar surface, such as a floor, a ceiling, and/or walls of a room, has adequate area to enable reproduction of low audible frequencies. The low frequency loudspeaker has an effective low end cut-off frequency of 55 Hz.
U.S. Pat. No. 4,210,223 teaches a low frequency loudspeaker apparatus includes a folded exponential horn which is divided to provide a bifurcated curved sound path from at least one electroacoustic transducer that is positioned at the throat of the horn to a volume into which sound waves are radiated that is located at the bifurcated mouth of the horn. The mean length of the folded exponential horn is such that, at an exponential rate of expansion between the throat and the bifurcated mouth, the area of the mouth is adequate for reproduction of low frequencies in the audible range. The low frequency loudspeaker apparatus has an effective low end cut-off frequency of 38 Hz. and affords 99 dB SPL output at three meters with one watt input which corresponds to about 20% efficiency measured in free space. Presence of a single boundary surface, such as a stage floor adjacent the mouth of the folded exponential horn, improves amplitude response by 3 to 6 dB.
A small dimension low frequency folded exponential horn loudspeaker has a unitary sound path for direction of acoustical waves from an electroacoustic transducer to a volume into which the acoustical waves are radiated.
High fidelity sound reproduction requires reproduction of low audible frequencies. W. B. Snow, "Audible Frequency Ranges of Music, Speech, and Noise," Jour. Acous. Soc. Am., Vol. 3, July, 1931, p. 155, for example, indicates that high fidelity sound reproduction of orchestral music requires that the frequency band should extend to as low as 40 Hz. It is well established that loudspeakers, in order to reproduce a given frequency range, must have dimensions based on the wavelength which corresponds to the lowest frequency in the range. In the case of one type of loudspeaker, the exponential horn loudspeaker, for example, the area of the exponential horn mouth is determined on the basis of the wavelength of the lowest frequency to be reproduced. At an early date, to obtain high fidelity sound reproduction with exponential horn loudspeakers, and, in particular, the inclusion of low audible frequencies, large exponential horn loudspeakers were constructed. For example, theater loudspeakers as large or larger than eight feet in length and four feet by four feet in transverse dimensions were built in order to obtain reproduction of low audible frequencies. Later, the outside dimensions of the exponential horns were reduced by folding, but even then the dimensions of the mouths were large for reproduction of low audible frequencies. More recently, folded exponential horn loudspeakers with reduced mouth dimensions have been used in proximity to boundary surfaces, such as a floor, a ceiling, and/or walls of a room, to increase the effective mouth area so that low audible frequencies are reproduced while at the same time the dimensions of the low frequency loudspeakers are minimized. See, for example, Sandeman, U.S. Pat. Nos. 1,984,550, 2,310,243 and 2,373,692, and Klipsch, "La Scala," Audio Engineering Society Preprint No. 372, April, 1965. The low frequency folded exponential horn loudspeakers, such as those which are disclosed in the above-cited references, have small dimensions and, when their mouths are located proximate planar surfaces, enable reproduction of low audible frequencies. However, each of these low frequency folded exponential horn loudspeakers is structurally complex due to the structure of the folded exponential horn which defines the sound path from the electroacoustic transducer to the volume into which sound is radiated. Perhaps the simplest construction appears in the above-cited Audio Engineering Society publication. In that construction, the folded exponential horn is bifurcated to define a double sound path. Due to the complex structure, the production of high fidelity, small dimension, low frequency folded exponential horn loudspeakers has required considerable craftsmanship. High quality control in manufacture has been necessary to assure that the construction meets specifications. Consequently, the cost of low frequency folded exponential horn loudspeakers has been high.
U.S. Pat. No. 5,212,732 teaches a loudspeaker system of the dipole type, particularly for use in surround sound, reverberation and similar applications. A speaker system includes a pair of woofers having dual voice coil drivers mounted on oppositely facing baffles (e.g., front and rear facing). Preferably, each baffle also includes a high frequency speaker mounted thereon. On a first baffle (e.g., front), both voice coils of the dual voice coil driver and the voice coil of the high frequency speaker are driven in-phase, and on the other baffle (e.g., rear), the second voice coil of the dual voice coil driver and the voice coil of the high frequency speaker are driven out-of-phase from those from the first baffle but in-phase with one another. The coils of the speakers are driven from suitable filter circuits.
U.S. Pat. No. 5,212,732 teaches a loudspeaker system of the dipole type, particularly for use in surround sound, reverberation and similar applications. The speaker system includes a pair of woofers having dual voice coil drivers mounted on oppositely facing baffles (e.g., front and rear facing). Each baffle also includes a high frequency speaker mounted thereon. On a first baffle (e.g., front), both voice coils of the dual voice coil driver and the voice coil of the high frequency speaker are driven in-phase, and on the other baffle (e.g., rear), the second voice coil of the dual voice coil driver and the voice coil of the high frequency speaker are driven out-of-phase from those from the first baffle but in-phase with one another. The coils of the speakers are driven from suitable filter circuits. Various forms of loudspeaker systems have been developed, and the types of speakers as well as the technologies involved pertaining to woofers, tweeters, mid-range and other forms of speaker systems are well known. Stereo sound systems using front speakers with or without some form of woofer or subwoofer, along with rear and/or side speakers, have become prevalent particularly for sound systems used to reproduce sound in "home theater" video systems for playing back video motion pictures and similar program material. The typical installation comprises a pair of front speakers positioned to either side of the TV screen, preferably with a center speaker and/or a subwoofer, and along with a pair of right and left side speaker and/or a pair of left and right rear speakers.
An Audio Engineering Society (AES) paper entitled "New Factors in Sound for Cinema and Television" by Tomlinson Holman, presented at the 89th Convention of the Audio Engineering Society, Los Angeles, Calif., Sep. 21-25, 1990, and reprinted in the Journal of the AES, Volume 39, No. 7/8, (preprint #2945) notes that the best directivity pattern for the "surround" loudspeakers is not the conventional forward radiating direct radiator, but rather dipolar radiation with the principal lobes of the dipole pointed, not at the listening area, but at the room surfaces with the null in the radiation pattern pointed at listeners, and that the best surround loudspeaker is physically invisible.
U.S. Pat. No. 4,733,749 teaches a loudspeaker system for low frequencies has a manifold chamber into which oppositely mounted and aligned woofer units radiate sound. The chamber radiates the sound perpendicularly to the woofer axes, either directly into space or into a horn. An additional back woofer may radiate directly in the perpendicular direction. An arrangement of speakers for a low-frequency sound reproduction is system particularly adapted for high power output and has manifold for coupling multiple low frequency loudspeakers, in a single sound-radiating enclosure. Multiple loudspeakers are often used in sound applications requiring high acoustic power output (sound volume), such as in theaters or arenas, or for studio and stage monitoring, discotheques and the like. In many sound systems, several components, such as driver/horn assemblies or cone/enclosure loudspeakers, are used for sound reproduction across the entire range of audible sound, with different devices covering the bass (low-frequency), midrange and high-frequency portions of the sound spectrum. Low-frequency speakers are customarily referred to as "woofers". A particular sound application may require an especially high power output across the whole audio spectrum. With respect to the low-frequency range, this has been accomplished in the past, in general, by increasing the number of loudspeakers, because of the need to set large volumes of air in motion to create high acoustic power. In order to move large air volumes, the excursion of a moving diaphragm having a given cone area could be increased, but since acoustic distortion increases with increasing excursion once the linear limitation of the loudspeaker suspension is reached, the solution of using multiple loudspeakers is generally preferred. Multiple loudspeakers are conventionally mounted on a front baffle board of a speaker housing or enclosure. The housing may be closed, or may be provided with one or more phase-inverting ports or ducts (as in a bass-reflex type enclosure). Acoustic coupling and addition occurs in such structures at frequencies where the wavelengths are sufficiently greater than the distances between the individual speakers or phase-inverting ports.
U.S. Pat. Nos. 4,391,346 and 4,437,540 teach individual speaker units which are set in the walls of a cavity behind a front baffle board. The speaker units are arranged so that the sound-radiating axis of each speaker unit angularly converges on and is concentrated on a point of the central axis of the cavity, just behind the front baffle, toward which the speakers are generally aimed. While such an arrangement may improve mid-range sound reproduction, low-end frequency reproduction is adversely affected, as the cavity behaves like a short acoustic horn having a rapid flare rate, such a horn being incapable of sustaining very low-frequency sounds. A maximum output speaker system for high-volume sound. A more specific object is to provide an efficient arrangement for summing the outputs of a number of individual low-frequency speakers for radiation from a single sound-radiating aperture. The maximum output speaker system minimizes destructive sound interference and maximizes coupling between loudspeakers at low frequencies. The sound-radiating axes of the individual speaker units are not aimed towards the chamber exit. Instead, pairs are aimed directly at or away from each other. This optimizes low frequency performance without peaking medium-pitch sound. The manifold chamber exit is smaller than the sum of the diaphragm areas of the individual speakers inside the chamber.
U.S. Pat. No. 3,903,989 teaches a loudspeaker system which has a cabinet with two compartments, a first of which contains a low-frequency loudspeaker for producing an omnidirectional radiation pattern, and the second compartment, above the first, containing a rotationally adjustable vertically oriented baffle on which are supported additional loudspeaker motors designed to cover the mid- and high-frequency bands of the audio frequency spectrum. The baffle is so shaped and the additional loudspeaker motors located in positions thereon that they operate as high-efficiency gradient or dipole loudspeakers over a significant portion of their respective frequency ranges, whereby the directivity of the loudspeaker system can be controlled by adjustment of the position of the baffle relative to the cabinet. It is conventional in loudspeaker systems to divide the audio frequency range of interest between a plurality of individual loudspeaker drivers mounted in a common enclosure, the higher quality systems utilizing a low frequency driver, or "woofer" for the very low frequencies, a smaller driver for the lower mid-range of frequencies, a still smaller driver for upper mid-range frequencies, and one or more "tweeters" for the high-frequency range. Because the wavelengths of the mid-and high-frequency signals are shorter than those of the low frequency signals, the directivity of the mid- and high-frequency signals of any particular drive is sharper than that of the low frequency signals. Accordingly, the sound field produced by an output signal from a given loudspeaker driver is increasingly narrower with increase in the signal frequency, with the consequence that the mid- and high-frequency signals are severely attenuated in directions offset greater than about 30.degree. to 60.degree. from the central axis of the loudspeaker array, depending on the dimensions of the driver and the frequency of the signal. The nature of this problem is described in detail in a paper by applicant entitled "Broadening the Area of Stereophonic Perception" which appeared in the Journal of the Audio Engineering Society, Vol. 8, No. 2, pp. 91-94 (1960), and a loudspeaker arrangement representing a solution to the problem is described and claimed in U.S. Pat. No. 3,080,012. The problem as it applies to quadraphonic reproduction is described in a paper entitled "Quadrophony Needs Directional Loudspeakers" which appeared in the March 1973 issue of Audio Magazine, pages 22, 24, 26 and 30.
U.S. Pat. No. 4,437,541 teaches a controlled dispersion loudspeaker configuration in which a loudspeaker is mounted through a hole in a front baffle forming a seal between the speaker and the baffle. A rear baffle is parallely spaced a predetermined distance away from the front baffle by means of spacers. Acoustically absorptive material is placed between the two baffles and is acoustically open on at least two opposite sides. The sound waves from the rear of the speaker exit from the acoustic material and serve to cancel the sound waves at the sides and rear of the loudspeaker configuration eminating from the front of the speaker. The size of the baffles, as well as the spacing therebetween, bears a particular relationship to the frequency of the sound to be reproduced by the loudspeaker.
The inventor incorporates the teachings of the above-cited patents into this specification.